1. Field of the Invention
The present invention relates to an optical recording medium having a semi-transparent recording layer, and a method and a device for recording data on the optical recording medium.
2. Description of the Related Art
There are known double-layer phase-change optical recording media having two phase-change recording layers L0 (rear side layer) and L1 (front side layer).
Optical recording media such as DVD (Digital Versatile Disc) use a modulation scheme to encode data to be recorded into recording marks along each track of the recording layer. The length of recording marks is nT, where n is an integer and T is one clock cycle. CD-RW (Compact Disc Rewritable), for example, uses the Eight-to-Fourteen Modulation (EFM) scheme to encode data to be recorded into recording marks with lengths ranging from 3 T to 11 T.
Rewritable optical recording media such as CD-RW and the like generally have a phase-change layer as a recording layer. A laser beam is modulated in accordance with the data to be recorded and the obtained laser beam pulses are irradiated onto the recording layer to form recording marks with various lengths nT. The recording marks in the phase-change layer are in the amorphous state and the spaces between the marks are in the crystalline state.
The amorphous recording marks are formed by melting the crystalline part with laser beams and quenching it quickly, while the crystalline state is obtained by irradiating laser beams thereon at above the crystallization temperature of the phase-change layer for more than a set period of time.
The pulse series of the laser beam to obtain the amorphous state includes write pulses of a write power and pulses of a bias (almost bottom) power, and the pulse series to obtain the crystalline state includes pulses of an erase power level.
To write an nT recording mark in a CD-RW, (n−1) write pulses are emitted. For example, four write pulses (5−1=4 pulses) are emitted to write a 5 T recording mark.
To control the mark length, for example, to increase the mark length by 1 T, e.g., from 5 T to 6 T, a pulse series of 1 T period consisting of a write pulse and a bias level pulse is added. The width Tw of the added write pulse is 0.2 T or more, taking account of the rise time and the fall time of the pulse. This means that the interval between write pulses is (1−Tw)T and less than 0.8 T. If the rise time and the fall time are reduced, Tw becomes less than 0.2 T, while (1−Tw)T is less than 1 T but infinitely close to 1 T.
With conventional recording strategies, the trailing edges of recording marks tend to be indistinct because of heat accumulated in the recording layer before the last write pulse. This is more evident in recording layers that are slow cooling structure because of thin metal heat-sink layers (to be described later). Conventionally, a cooling pulse was added after the last write pulse and before the laser power was returned to the erase power level to adjust the position of the trailing edge of the recording mark. This approach is effective only to the limited extent with slow cooling structured recording layers and cannot solve the problem that various margins are lost.
Meanwhile, with the increase in data storage density, multilayer optical recording media have been developed. The recording layer on the light incident side of such optical discs needs to be semi-transparent in order to enable recording and reproducing to and from the recording layer below. Accordingly, in optical recording media having phase-change recording layers the above-mentioned metal heat-sink layer which was conventionally about 100 nm thick needs to be reduced to, for example, less than 30 nm, so as to make the recording layer on the light incident side semi-transparent. The recording layer on the light incident side is thus slow cooling structure because of the reduced heat-sink effect.
This slow cooling structure brought about the problem of heat interference between consecutive recording marks and between adjacent tracks (so-called as cross erase), because the marks that were transformed into the amorphous state by rapid cooling are not cooled sufficiently quickly.
As the heat interference between recording marks and cross erase between tracks were problems to be solved not only for the multilayer optical recording media but also for single layer recording media, various solutions have been proposed as shown in Japanese Patent Laid-Open Publications Nos. 2003-203337 and 2003-208713.
Japanese Patent Laid-Open Publication No. 2003-203337 shows a method of adjusting the level of erase power in accordance with the space length of the region to be erased. Japanese Patent Laid-Open Publication No. 2003-208713 shows a method wherein the write pulse power is switched between two levels.
However, these methods require adjustment of laser pulse at an increased number of power levels, making the laser modulation process complicated.
A 2 T strategy could be applied in which the pulse is synchronized with a 2 T clock so as to increase the cooling pulse before the last write pulse. In this case, however, the control of the mark length and width would be difficult, as the same number of write pulses are used to write marks with different lengths, e.g., m write pulses for both 2mT mark and (2m+1)T mark. Moreover, the pulse width needs to be increased to write long marks as the number of write pulses is small (e.g., three, for forming a 7 T mark). The large pulse width causes a wider area to be molten when recording, making the cross erase phenomenon more distinct, particularly in slow cooling structured recording layers.